1. Field of the Invention
The present invention relates to power regulation and, in particular, to the regulation of power applied to loudspeakers.
2. Background
In many applications, it is desirable to drive a loudspeaker as loudly as possible, without causing audible distortion or damage to the loudspeaker. Loudspeakers convert electrical energy into acoustic energy and thermal energy. When alternating electrical energy (power) is applied to the leads of a loudspeaker voice coil, forces are created which interact with the magnetic field in a magnetic gap. For example, in the conventional loudspeaker 10 of FIG. 1, the voice coil 12, which is attached to the coil form 14, will move in and out of the magnetic gap 16 in response to the alternating power being applied. This interaction may result in cone motion (a.k.a. excursion). Because the voice coil is rigidly connected to the cone 18, the cone 18 and the loudspeaker spider 20 will move with the same motion as the voice coil 12, thereby producing sound. The voice coil 12, as shown in FIG. 1, is situated between a front plate 22 and a pole piece 24 in the magnetic gap 16. In the typical loudspeaker design, the magnet 26 is held into place between front plate 22 and a back plate 28. Moreover, the vent hole 30 allows heat to dissipate, while the dust dome 32 protects the voice coil area from potentially harmful debris.
Virtually all loudspeaker damage results from the application of excessive power to the loudspeaker. Loudspeakers are highly inefficient and convert most of their applied power into heat. A significant component of this heat, which is a function of wasted power over time, is generated in the voice coil 12. This heat is transferred to the surrounding parts of the loudspeaker, mostly through conduction and convection.
Types of loudspeaker damage include thermal degradation, thermal failure, mechanical failure or a combination thereof. Thermal degradation results from the repeated application of excessive power over a period of time. Repeated overheating of the voice coil 12 leads to damaging thermal-expansion effects and material fatiguing. Such thermal degradation can further lead to thermally-induced mechanical failure since the materials that comprise the voice coil 12 and related components tend to become brittle and are generally more vulnerable to mechanical shock. In addition, heating of the voice coil will cause heating of the surrounding materials; overheating of the magnet 26 can irreversibly change its magnetic properties, especially magnets made of rare-earth materials such as neodymium.
Thermal failure, like thermal degradation, results from the negative heat effects of applying excessive power to a loudspeaker. However, instead of causing gradual degradation, a strong power surge can lead to a catastrophic thermal failure in which the voice coil 12 is heated to the point where it or other components of the loudspeaker literally melt, break or burn.
On the other hand, mechanical failure may occur when excessive power moves the voice coil 12 far enough that it strikes the back plate 28 or separates from the coil form 14. Similarly, the application of excessive power can cause the voice coil 12 to put excessive stress on the cone 18 or spider 20, causing tearing. In any of these cases, the voice coil 12 may become misaligned since the cone assembly is not suspended properly. It is this voice coil misalignment or cone/spider tearing that will lead to mechanical failure.
Existing methods of power limiting include measuring the voltage applied to the loudspeaker and limiting the power based on assumptions regarding loudspeaker impedance. However, such methods do not effectively limit the power delivered to the loudspeaker.
In U.S. Pat. No. 4,233,566, Nestorovic discloses a method of limiting power to a loudspeaker based on the assumption that the loudspeaker is a fixed resistive load. However, loudspeaker impedance is not simply a resistive load, but rather varies with frequency and driver temperature. Therefore, the power delivered to the loudspeaker cannot be accurately determined by assuming the loudspeaker is a fixed resistive load.
In U.S. Pat. No. 4,327,250, von Recklinghausen describes a method of limiting that uses a model of the loudspeaker. The voltage present across the loudspeaker terminals drives the loudspeaker model, and the output of the model is compared to a threshold. However, limiting is based solely on the output of the loudspeaker model, which is merely an estimation and not a measurement of power or voice coil temperature.
In U.S. Pat. No. 4,216,517, Takahashi describes a method of circuit protection for a power amplifier in which both voltage and current are detected. This method attempts to protect the amplifier from damage when the load impedance is too small. It does not protect the loudspeaker from excessive power or distortion.
In U.S. Pat. No. 5,719,526, Fink describes a method in which both the voltage and current applied to a loudspeaker are measured, and power is calculated as a function of frequency. A record of measurement data is stored and a Fourier transformation performed, where the minimum length of this record is dictated by the lowest frequency of interest in the signal. For audio, this lowest frequency is 20 Hz, or a minimum record length of 50 milliseconds. The length of this record severely limits the response time of processing that may occur on the input signal as a result of the measured power output.
In U.S. Pat. Application No. 2002/0118841, Button describes a system that limits the temperature of a loudspeaker's voice coil by estimating the voice coil temperature and comparing it to a predetermined threshold. This system uses the input signal and a thermal model of the voice coil to estimate the voice coil temperature. However, a thermal model is inherently susceptible to inaccuracy, because it is an estimation of the voice coil temperature and not an actual measurement.
Accordingly, there is a need in the technology for apparatus and methods that overcome the aforementioned problems.